Field of the Invention
The invention lies in the automotive field. More specifically, the invention relates to a method for monitoring the emission control system of a spark ignition internal combustion engine provided with a lambda-controlled catalytic converter. A so-called pre-catalyst lambda probe is arranged upstream of the catalytic converter in the exhaust gas flow direction, and a post-catalyst lambda probe is arranged downstream of the catalytic converter. The fuel supply system of the internal combustion engine is controlled by master control of the post-catalyst lambda probe in such a way that the signal of the pre-catalyst lambda-probe performs an oscillation about the value lambda .lambda.=1.
In an emission control system having two lambda probes, a pre-catalyst lambda probe (also referred to as a pre-cat lambda sensor) is used upstream of the catalytic converter as a measuring probe. A post-catalyst lambda probe (also referred to as a post-cat lambda sensor) is used downstream of the catalytic converter as a monitor probe for monitoring and compensating a static or dynamic lambda shift of the pre-catalyst lambda probe signal which would lead to an increase in emissions. Usually, the two lambda probes have two step action and their emitted voltage signal is, as in all lambda probes, dependent on the residual oxygen contained in the emissions. The oxygen content in the emissions depends in turn on the air-fuel mixture which has been fed to the internal combustion engine. In the case of a leaner mixture (.lambda.&gt;1), the output voltage of the lambda probe is usually below 100 mV, but in the region of range .lambda.=1 it changes virtually instantaneously and extends beyond 0.9 V in the case of a rich mixture (.lambda.&lt;1). This effect is referred to as two step action.
The dynamic and static properties of the pre-catalyst lambda probe are changed as a result of aging of the probe and contamination. As a result, the control point of the lambda control is shifted. For example, contamination with phosphorous can lead to an asymmetrical change in the probe response time and thus to a shifting of the probe control in the lean direction outside the optimum lambda range for the catalytic conversion. As a result, for example, the NO.sub.x emission may rise beyond a permitted limit. The post-catalyst lambda probe is used as a monitor probe for monitoring the catalytic conversion and for the fine control of the mixture so that the lambda value which is most favorable for the conversion can always be maintained. This is referred to as master control.
However, a functional diagnosis of the pre-catalyst lambda probe is possible only to a limited degree with the prior art. In particular, it is not possible to determine for how long the optimum lambda range has not been maintained, and how large an associated increase in emissions is.